1. Field of the Invention
The present invention relates to a semiconductor element in which an oxide semiconductor is used for an active layer, a method for manufacturing the semiconductor element, and a semiconductor device including the semiconductor element.
In this specification, a “semiconductor element” refers to an element that can function by utilizing semiconductor characteristics, such as a transistor or a diode. Further, a “semiconductor device” refers to all the devices that can function by utilizing semiconductor characteristics of the semiconductor element; an electronic display device, an electro-optical device, and a memory device are all included in the category of the semiconductor device.
2. Description of the Related Art
Transistors in which an active layer is formed using a semiconductor thin film which is formed over a substrate having an insulating surface are widely used in electronic devices such as integrated circuits (ICs) and image display devices (display devices).
Conventional thin film transistors mostly have what is called a planar structure, in which a semiconductor layer, an insulating film, an electrode, and the like are stacked over a plane. With advances in manufacturing processes which enables miniaturization of such transistors, various problems such as increases in short-channel effect and leakage current arise. Therefore, in recent years, a transistor having a novel structure which is an alternative to the conventional planar structure has been developed. For example, Patent Document 1 discloses a fin-type transistor in which a polycrystalline silicon film is used as an active layer (referred to as a semiconductor thin film in Patent Document 1).